1. Technical Field
This invention relates to a pneumatic actuator for rotating an output shaft by transforming a linear motion (reciprocating motion) generated by compressed air into a rotary motion. More particularly, this invention relates to a pneumatic actuator to be applied to a ball valve, a butterfly valve or other similar valve that switches a valve body, such as a ball or a disk, by a rotation of about 90°.
2. Description of the Related Art
The conventional pneumatic actuator for use in the valve has adopted such a structure as to impart a switching motion to the valve body of a rotary valve, such as a ball valve or a butterfly valve, by converting a linear motion (reciprocating motion) into a rotary motion through a built-in motion-transforming mechanism and causing the turning force produced in an output shaft in consequence of the transformation effected by the aforementioned motion-transforming mechanism to a stem connected to the output shaft. The motion-transforming mechanism that is built in this actuator is known in the scotch yoke type and the rack-and-pinion type. Particularly, most actuators of small sizes adopt the rack-and-pinion type motion-transforming mechanisms.
Incidentally, the rack-and-pinion type motion-transforming mechanisms generally have a structure comprising a rotatably disposed output shaft furnished in a cylinder with a pinion, a pair of pistons disposed in the cylinder and adapted to produce a reciprocating motion therein, and racks fitted each to the pistons so that the reciprocating motion of the pistons may be transformed into a rotary motion by virtue of the mesh to be formed between the racks and the pinion.
In the rack-and-pinion type pneumatic actuators mentioned above, the inclinations of the pistons during the operation thereof are ordinarily regulated on the basis of the effective diameters of the pinions and the racks of the pistons by the sliding surfaces occurring between the outside diameters of the pistons and the inside diameter of the cylinder and particularly the pistons themselves during their reciprocating motions tend to produce a phenomenon of inclining in the axial or rotary direction thereof. These actuators, therefore, have entailed the problem of suffering the pistons to sustain fracture and scouring their surfaces even to an extent of degrading their output efficiency.
The following techniques have been heretofore proposed as measures to cope with these problems. JP-A-SHO 53-103915 (hereinafter referred to as “Patent Document 1”) discloses a structure which, with the object of coping with the problem, precludes the phenomenon of the inclination of the pistons during the reciprocating motion thereof by having pistons provided one each at the opposite terminals of a piston rod furnished with rack teeth to be meshed with a pinion and causing an annular body fixed with a key to the shaft to be brought into rolling contact with a travel route fastened to the supporting parts of the rack teeth.
Utility Model Registration No. 2561362 (hereinafter referred to as “Patent Document 2”) teaches an actuator that is provided along the longitudinal direction of a rack surface with a guide rail and has this guide rail disposed so as to be freely guided on the outer peripheral surface of an output shaft.
The prior art references mentioned above concern techniques that, in actuators adapted to switch a valve with compressed air and a compression spring respectively as a power source, make it possible to alleviate the phenomenon of the inclination of the pistons used therein toward the axial directions and the rotary directions thereof. The technique disclosed in Patent Document 1 requires an annular part for contact with the rack. When the actuator is in such a structure as to induce deviation of the output shaft from the center of the cylinder, the annular body mentioned above is inevitably suffered to interfere with the inside diameter of the cylinder of the actuator and, to avoid this trouble, is required to be replaced with a contacting member of a special shape.
Then, the technique disclosed in Patent Document 2 requires the guide rail for supporting the rack to be fixed to the actuator body. Thus, this technique results in adding to the number of component parts and complicating the structure of the actuator.
Further, in the case of a high-output actuator of the spring return type, though the compression spring thereof is fated to assume a large size and generate a large resilient repulsive force, this compression spring is formed in a helical shape and the terminal part of this compression spring is consequently enabled to generate in increasing prominence a retracting action in the circumferential direction of the spring diameter in consequence of the expansion and contraction of the spring. Moreover, since the terminal part of the compression spring is pressed against the piston, the retracting action of the terminal part of the compression spring has the possibility of rotating the piston and causing the rack disposed integrally on the piston to tilt and unevenly collide against the pinion disposed integrally on the output shaft and consequently curtailing the service life of the actuator.
Patent Document 2 discloses a concept of providing a blocking plate disposed integrally on the terminal part of a cylinder containing a compression spring with an air vent that is adapted to admit air into the cylinder and discharge air from the cylinder.
This air vent indeed is disposed at such a position as to avoid the terminal part of the compression spring. The air vent has the possibility of being blocked by the terminal part of the compression spring when the actuator is miniaturized. The air vent also has the possibility of leaking rainwater therethrough when the actuator is used outdoors.
This invention has been developed in light of the various problems encountered by the prior art. It is aimed at providing a pneumatic actuator which is capable of preventing a piston from developing a phenomenon of inclining in the axial or rotary direction, enabling the reciprocating motion of the piston to be stably maintained for a long time, facilitating an inspection for confirming the property of sealing the cylinder and the piston and allowing the inspection to be infallibly performed quickly as well, and enabling the inspection of the operation and the inspection for detection of outer leakage to be simultaneously carried out.
It is further aimed at providing a pneumatic actuator which even in a combined operation type structure avoids suffering a compression spring to pop out during an overhaul, allows the aperture of the actuator to be adjusted as well on the compression spring-containing side, permits the interior of a cylinder containing the compression spring to be provided with an air vent incapable of leaking rainwater, and forms only a few parts projecting from the external shape of the actuator.